Anyone who has ever, at any time, tried to explain something to someone else, is aware of the "light bulb moment".
The light bulb moment is a moment of sudden inspiration, revelation, understanding or recognition. It's such a common experience that it has made its way into cartoon iconography, with the image of a light bulb lighting up above a character's head when he or she suddenly "gets it".
You can see the light bulb in real life - you can see the moment when understanding dawns - nicely illustrated in the photo here from Flickr (mouse-over for credits). It's a brightening of the features, an increase in the level of engagement, stiffening body posture, a widening of the eyes, a smile. Those are the outward signs of the inward dawning of comprehension.